The heart of a computer's long term memory is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider toward the surface of the disk, and when the disk rotates, air adjacent to the disk moves along with the surface of the disk. The slider flies over the surface of the disk on a cushion of this moving air. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic transitions to and reading magnetic transitions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
Data can be written to the magnetic media by a write head that includes a magnetic write pole having a small cross section at the air bearing surface, and a magnetic return pole having a larger cross section at the air bearing surface. An electrically conductive write coil generates a magnetic field that causes a magnetic flux to flow through the write pole and return pole. The small cross section of the write pole allows a dense, strong write field to emit from the tip of the write pole toward the magnetic medium where it magnetizes a high magnetic coercivity top layer on the magnetic media. The resulting magnetic flux then travels through a magnetically soft under-layer of the magnetic media, to return to the write head at the return pole, where it is sufficiently spread out and weak that it does not erase previously recorded bits of data.
A magnetoresistive sensor such as a GMR or TMR sensor has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, or barrier layer, sandwiched between first and second ferromagnetic layers, referred to as a pinned layer and a free layer. First and second leads are connected to the sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
In current magnetic recording systems, the height at which the slider passes over the magnetic media (fly height) is very small and is a critical parameter for the performance of the recording system. In order to control the height of the read and write heads over the media at these extremely low fly heights, the recording system can incorporate thermal fly height control. In such a system a heating element can be placed near the read and write heads to locally and controllably heat the area around and within the read and write head. When heated, thermal expansion causes the read and write heads to protrude by a desired, controllable amount. However, such thermally controlled fly height systems present their own challenges, in that the heat from the heating element can adversely affect the performance of the magnetic sensor.